One of the most common discoveries for new and old aquarists are crabs in live rock. There is a saying some aquarists use, and that is "there is no such thing as a good crab". This doesn't hold true. Whilst there are some crabs which can do alot of damage throughout an aquarium, they are not ALL bad. Below you will find some pictures to illustrate the theory most aquarists now use to differentiate "good" from "bad".
Some crabs live well with other organisms, including algae, anemones and coral. These are generally bright colours, smooth outer shell, and small (relative to body size) claws. Some of the main pincers can also resemble fans, for filter feeding.
For example, some crabs create a burrow (non-farmful to the coral) and reside within the coral itself, while it grows around the crab. as seen in the picture, these crabs tend to be filtering crabs.
Bad crabs can often be dark in colour, more often brown, covered in small fine hairs and with large main pincers. Often seen among live rock rather than other organisms.
Crabs often reproduce in aquaria, or show the process of breeding. It is unlikely their offspring will survive, due to the fact most crustacea (including crabs) have a long life history, with various stages between egg and settlement. Female crabs become "berried", with a mass of eggs being stored and fanned underneath their main carapace. Upon release, the crab stands in a high flow area, and "shakes" the eggs loose whil standing on their back legs. These eggs will likely be consumed by corals, fish or other invertebrates, or be taken out of suspension through filtration equipment (e.g. skimmers, filter socks and pumps).
Removing crabs can be easy, or a very big pain. There are many tips and tricks though, to make it easier.
- Baiting. There are commercial products to capture crabs, or you can use a drinking glass. Place bait (usually in the form of something marine based, such as squid, prawn etc.) and weight this in the bottom of the glass. Make sure you lean the glass next to liverock. Once the crab falls in to get the bait, the slippery surface of the glass will not permit it to exit. It is important to use glass, a crabs sharp feet can sometimes climb plastic.
- Skewer. Its relatively simple. If you can identify the crabs hiding hole, simply skewer the crab.
- DO NOT COOK LIVEROCK. "Cooking" liverock (LR) by removing a piece and placing it in freshwater, or altering the temperatrure of the water either via boiling or cooling, to encourage a crab to exit its tight hole may work, but it does more harm than leaving the crab in place. Cooking damages all life on the rock, and this will die off extremely quickly once placed back in the tank. If you find no other method will work, consider removing the rock and breaking it apart before cooking.
- Fish control can be effective, but the fish that can and will readily consume crabs will also consume most other invertebrates, good or bad. Trigger fish and eels are often considered the most effective.
Commensal crab in Pocillopora damicornis colony.
- Home Breeders FAQ For Marine Invertebrates by Rob Toonen
- Aquarium Invertebrates: Crabs in the Marine Aquarium by James W. Fatherree - Advanced Aquarist's Online Magazine